1. Field of the Invention
The present invention relates to a DC—DC converter, and particularly to a DC—DC converter which is used in a DC power source circuit and converts a power source voltage of a DC power source into a different DC voltage.
2. Description of the Related Art
For example, one example of a DC—DC converter used in a DC power source circuit is shown in FIG. 18.
The DC—DC converter shown in FIG. 18 comprises a pair of conversion circuit parts 1, 2, two transformers Tr1, Tr2, a pair of rectification circuit parts 3, 4, and an LC smoothing circuit part 5. The pair of conversion circuit parts 1, 2 have two pairs of switching elements Q1, Q4, Q2, Q3 and Q5, Q8, Q6, Q7 which are connected by a full bridge configuration. The two transformers Tr1, Tr2 are connected to the output sides of the conversion circuit parts 1, 2. The pair of rectification circuit parts 3, 4 are connected to secondary side outputs of the transformers Tr1, Tr2 and are made of two pairs of diodes D1, D4, D2, D3 and D5, D8, D6, D7. The LC smoothing circuit part 5 is connected in common with the output sides of the rectification circuit parts 3, 4. In this DC—DC converter, a pair of the conversion circuit parts 1, 2 are connected in parallel with a DC power source E. Also, series capacitors C1, C2 are inserted and connected between the output sides of each of the conversion circuit parts 1, 2 and the primary sides of the transformers Tr1, Tr2.
In this DC—DC converter, the switching elements Q1, Q4, Q2, Q3 and Q5, Q8, Q6, Q7 of the conversion circuit parts 1, 2 are alternately turned on or off to obtain an AC waveform output. This AC waveform output of the conversion circuit parts 1, 2 is transformed by the transformers Tr1, Tr2 and the secondary side outputs of the transformers Tr1, Tr2 are rectified by the rectification circuit parts 3, 4 and also are smoothed by the LC smoothing circuit part 5. Therefore, a desired DC voltage VOUT is generated (for example, see JP-A-2002-223565).
By the way, as means for increasing a conversion capacity of the DC—DC converter described above, means using a semiconductor device with large voltage and current or a large-size transformer in components forming the DC—DC converter can be implemented.
However, general versatility of the DC—DC converter decreases as a conversion capacity of a DC—DC converter single device providing the configuration shown in FIG. 18 is increased. That is, in the case of a DC—DC converter with a small single device capacity, it can cope with various capacities requiring a capacity larger than its small capacity and has flexibility, but in the case that a single device capacity is large, it becomes difficult to cope with applications requiring a capacity smaller than its large capacity.
Also, in the case of using a semiconductor device with large voltage and current in components forming the DC—DC converter, it was very difficult to obtain a semiconductor device with high speed and high performance, or in the case of using a large-size transformer, it was difficult to obtain things with good core characteristics.
Thus, the means for increasing a single device capacity of the DC—DC converter is undesirable because there are the problems described above. Therefore, as another means for increasing a conversion capacity of the DC—DC converter, it is considered that plural unitary units are connected in parallel with a DC power source E using a circuit configuration of the DC—DC converter shown in FIG. 18 as a unitary unit.
For example, a DC—DC converter having two unitary units A1, A2 which are connected in parallel with a DC power source E is shown in FIG. 19.
The unitary unit A1 of the DC—DC converter shown in FIG. 19 provides a following configuration. That is, a pair of conversion circuit parts 11, 12 for converting a power source voltage of the DC power source E into an AC by switching elements Q11 to Q14 and Q15 to Q18 of a full bridge configuration are connected in parallel with the DC power source E. Rectification circuit parts 13, 14 made of diodes D11, to D14 and D15 to D18 are provided to the output sides of each of the conversion circuit parts 11, 12 through transformers Tr11, Tr12. Series capacitors C11, C12 are inserted and connected between each of the conversion circuit parts 11, 12 and the transformers Tr11, Tr12.
Also, the unitary unit A2 has the same circuit configuration as that of the unitary unit A1. That is, it comprises conversion circuit parts 21, 22 of parallel connection made of switching elements Q21 to Q24 and Q25 to Q28 of a full bridge configuration, and rectification circuit parts 23, 24 made of diodes D21 to D24 and D25 to D28 connected to the output sides of each of the conversion circuit parts 21, 22 through series capacitors C21, C22 and transformers Tr21, Tr22. Incidentally, an LC smoothing circuit part 15 is provided to the output sides of the rectification circuit parts 13, 14, 23, 24.
When the plural unitary units A1, A2 are connected in parallel with the DC power source E thus, without respectively increasing conversion capacities of the unitary units A1, A2 and using a semiconductor device with large voltage and current or a large-size transformer as components, a conversion capacity of the whole DC—DC converter can be increased.
However, in the DC—DC converter shown in FIG. 19, when the conversion circuit parts 11, 12, 21, 22 which are voltage sources are connected in parallel, across current flows between the conversion circuit parts by a slight difference in output voltages of the respective conversion circuit parts 11, 12, 21, 22. An output current of the DC—DC converter increases or decreases by this cross current and an unbalance state occurs and thereby overcurrent occurrence is caused. In order to equalize the output voltages of each of the conversion circuit parts 11, 12, 21, 22, a method for performing a feedback control of the output voltages of the conversion circuit parts 11, 12, 21, 22 or a method for suppressing a difference in output voltages by improving accuracy between components in each of the conversion circuit parts 11, 12, 21, 22 is considered. However, bad effects that a control circuit becomes complicated in the former method and an increase in component costs is caused in the latter method occur respectively.
In the above-mentioned DC—DC converter as shown in FIG. 19, the DC—DC converter has a pair of conversion circuit parts, rectification circuit parts, transformers and series capacitors as a unitary unit, but it is not limited to this. An inverter unit IU can be considered as a unitary unit. That is, as shown in FIGS. 18 and 20, the inverter unit IU has an inverter board 60 and a pairs of transformers Tr1, Tr2. The inverter board 60 mounts a pair of conversion circuit parts 1, 2 thereon (In FIG. 20, eight switching elements Q1 to Q4 and Q5 to Q8 are shown). Primary sides of the pairs of transformers Tr1, Tr2 are respectively connected to outputs of each of the conversion circuit parts 1, 2 of the inverter board 60 via lead wires 70, 80, and secondary sides thereof are respectively connected to inputs of rectification circuit parts 3, 4 via lead wires 90, 100.
In this DC—DC converter, in order to increase a conversion capacity of the DC—DC converter, it is considered that plural inverter units IU, each having the inverter board 60 mounting the pair of conversion circuit parts 1, 2 and the pair of transformers Tr1, Tr2, are connected in parallel with a DC power source E while the plural pairs of transformers Tr1, Tr2 of the inverter units IU are connected a pair of the rectification circuit parts 3, 4.
However, if the plural inverter units IU are provided, the number of the transformers Tr1, Tr2 included in the inverter units IU are increased, and thus wiring work for connecting the secondary side of the plural transformers Tr1, Tr2 to the pair of the rectification circuit parts 3, 4 can be complicated. Further, since the inverter boards 60 and the transformers Tr1, Tr2 included in the inverter units IU are heat generation sources, a placement relation between inverter boards 60 and the transformers Tr1, Tr2 has to be considered in view of heat dissipation characteristics.